Wound Rotor Induction Machine - Laboratory Measurements of Stator Current Harmonics and Harmonic Propagation in the Distribution Grid Causing Power Quality Problem
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- Institutt for elkraftteknikk 
A great deal of parameters effects the power system quality and harmonics is one of them. The origin of this thesis is a quality issue experienced by a customer in Norway, where troublesome flicker in the light equipment occurred periodically. Former investigations have found the cause of the problem to be fluctuation of, especially, the 7th harmonic voltage. The reason for the fluctuation is the integer harmonic component and an interharmonic component, with frequency close to the integer, interfering with each other creating a beat frequency with approximately a frequency of 2 Hz. The quality problem coincided 100 % with a pump station located downstream in the distribution grid. The main objective of this thesis is to find if the wound-rotor induction machines (WRIM) in the pump station can be the only source of the interharmonic and harmonic components causing the flicker.From the literature study about stator harmonic current it is found that the two components can arise in a WRIM. Where the origin of the harmonic 350 Hz component is saturation and the interharmonic component is due to a rotor-stator interaction. An equation is found to calculate the frequency of the interharmonic component which is dependent of the slip of the machine.To verify the findings from the literature study, a laboratory test of an WRIM is performed at the Smart Grid laboratory at NTNU. The machine is connected to a controllable voltage supply, where both sinusoidal voltage and voltage containing harmonics is feed the machine to investigate the response in the current spectra with different supply condition. With results obtained from the analysis of data from the WRIM, a power system harmonic analysis is executed in the software Power System. Both a harmonic load flow and frequency sweep is carried out. This is done to investigate how the distribution grid will respond when a non-linear load (assuming the WRIMs are the only source of the power quality problem) is connected and to see how different parameters related to the simulation will affect a prospective resonance. The stator current spectra of the WRIM in the laboratory displays both the harmonic and interharmonic component, which is in consistency with similar papers on the subject. The 7th harmonic current is highly dependent on the voltage level as the component increases in the test where the voltage was increased above the machine s rated voltage. Adding harmonics in the supply voltage yields minor variations in the current spectra except the amplitude of the associated supply harmonics. In some of the tests low resolution in the DFT gives consequential unusable data.The power system harmonic analysis reveals that the harmonic currents from a non-linear load will propagate in the distribution grid, causing distortion of the supply grid. A worst case scenario, where only one load is connected and the resistance is set to be independent of the frequency, reveals a resonance close to 350 Hz when two shunts is connected to the bus where the harmonic current source is connected.